Electro-optical display devices are the object of considerable research efforts. Of the various display systems that have been developed, devices utilizing liquid crystals, in particular, have drawn commercial interest. For example, liquid crystal devices have found popular utility in display applications such as wrist watches, calculators, laptop computers, personal digital assistants, and the like.
Compositions characterized as liquid crystals include a wide range of materials. The different electrical and optical properties exhibited by these liquid crystalline materials make possible a number of mechanisms for light modulation. Such mechanisms include phase transitions, dynamic scattering, and field effects, all of which are well known in the art.
Field effect devices are of particular utility. The effect that is commercially most significant at present is the rotation of polarized light by a twisted nematic liquid crystal alignment and the disappearance of this effect when an electric field is applied across the device. Twisted nematic liquid crystal devices typically comprise a suitable liquid crystal composition confined between two optically transmissive plates, the plates having transparent conductive films affixed to their surfaces facing one another in the device. The alignment of the surface layers of the liquid crystal in the "off" state of the device is determined by the interaction of the liquid crystal composition with the confining surfaces of the display device. The orientation of the surface layers of the liquid crystal is propagated throughout the bulk of the composition. The nature of the alignment determines the off-stage optical properties of the device and the manner in which the liquid crystal molecules will be reoriented by an applied electric field.
To effect orientation of a confined liquid crystal, the internal surfaces of the conductive plates of a sandwich display device can be prepared by unidirectionally rubbing the surfaces prior to fabrication of the device. The liquid crystal molecules immediately adjacent to each rubbed surface tend to orient themselves in the same direction as the rubbing. By arranging the opposing conducting plates with the axis of the rubbed surface at, for example, right angles to each other, the liquid crystal molecules at points intermediate the two plates will orient themselves to a degree which is a function of the distance from the two plates. Accordingly, in this example, the liquid crystal will align itself in a continuous spiral path that twists through the 90.degree. angle between the opposing plates.
If the light-rotating liquid crystal "sandwich" is mounted between a pair of crossed light polarizer elements, polarized light will pass into the device and be rotated through a 90.degree. angle as it is transmitted through the twisted nematic crystal composition from one surface of the device to the other. Due to the 90.degree. light rotation effected by the twist of the liquid crystal, the polarized light will be set to pass through the second crossed polarizer mounted on the opposing side of the display. In the prior art, it is known that by positioning a light reflector behind the second polarizer, the polarized light can be reflected back through the second polarizer to pass through and be rotated by the confined liquid crystal and then exit out the first polarizer where it was introduced.
When an electric field is applied across the liquid crystal composition between the two conductive plates, the twisted orientation of the liquid crystal is obliterated as the molecules align themselves with the applied field. As the liquid crystal is untwisted, polarized light entering the device through the first polarizer will no longer be rotated 90.degree. as it is transmitted through the liquid crystal. Therefore, the non-rotated light will be unable to pass through the second polarizer which is set correspondingly crossed to the first polarizer. Selective application of voltages across discrete segments of the liquid crystal device can readily accomplish patterns of bright areas (no applied electric field, resulting in reflected light) and dark areas (applied electric field, resulting in no reflected light).
In conventional liquid crystal devices, backlighting and edgelighting are oftentimes the greatest source of power drain. To reduce the energy requirements of such devices, it has been found that displays adequately viewable under ambient light can be provided by replacing conventional light source elements with a brightly reflective holographic diffuser, the reflective holographic diffuser comprising either a volume phase reflection hologram diffuser, or --to accomplish achromaticity--the combination of a volume or surface holographic transmission diffuser and a reflection layer. See, U.S. Pat. No. 5,663,816 (Chen at al.) and U.S. Pat. No. 5,659,408 (Weyon). Although substantial differences exist among the varieties of liquid crystal display devices employing such passive holographic illuminating means, in respect to their operation generally, polarized ambient light passing through a liquid crystal display element is intercepted and reflected by the reflective holographic diffuser, the reflected light being retransmitted as diffused light toward and through the liquid crystal display element, typically with gain within a predetermined viewing angle.
While the commercial popularity of holographically-illuminated liquid crystal display devices continues to develop, desire to raise certain efficiencies in respect of the display's manufacture yet provides further avenues for improvement. In particular, in accomplishing gain and a desirable light diffusion pattern, the reflective holographic diffuser will characteristically manifest an optical asymmetry with a definite top-bottom orientation, an orientation that must be maintained in manufacture to accomplish operative predefined display parameters. However, the polarizer elements in LCD display devices also have a predefined operational axis.
Under current manufacturing processes, the rear polarizer and reflective holographic diffuser are incorporated separately, and consequently, compelling the adoption of separate alignment procedures. Aside from increasing the costs of manufacture, these additional procedures introduce the possibility of misalignment, hence, increasing the possible number of defective or otherwise unusable display units. Further, inasmuch as polarizing material is oftentimes supplied commercially on a supporting carrier substrate, the overall bulk of the finished product is correspondingly increased by the thickness of said substrate.